The invention is directed to refractometer apparatus for measuring principal refractive indices of solids and liquids, and more particularly to the automation of such method and apparatus with expanded capabilities of automatically scanning a specimen and computing refractive indices with commensurate additional functions of differing phase indication, mapping thin sections and the plotting of petrofabric diagrams, etc.
Geologists, and particularly petrographers, devote much of their efforts studying thin sections of specimens to obtain goals such as the identification of various minerals present in thin sections of both rocks and ceramic materials and the estimation of the relative volumetric proportions of the minerals in the thin sections, and to determine the orientation of the optical indicatrix for all crystals of each mineral if it is anisotropic and non-opaque in the thin section in order to prepare a petrofabric diagram showing the preferred orientation, or lack thereof. There is a need to perform all such functions automatically in less time than now currently achievable. At present the determination of the orientation of the optical indicatrix for crystals and the petrofabric study of rocks is so time-consuming for optically biaxial crystals that it is rarely attempted by petrographers.
The routine techniques of optical crystallography have remained substantially stagnant subsequent to the development of the double-variation method by Emmons in 1928. In contradistinction thereto, X-ray crystallography has been revolutionized by the use of high speed computers, sophisticated statistical methods, and automation and thus has flourished as opposed to the atrophy of optical crystallography. From the use of carefully adjusted Abbe-Pulfrich refractometers, researchers have been able to measure, with a precision of 0.0002, the three principal refractive indices of biaxial crystals (.alpha., .beta. and .gamma.). However, in this century mineralogists have gradually abandoned such techniques because they required a flat surface of appreciable area to be cut and polished on a crystal specimen. Therefore oil immersion techniques became increasingly popular because they were applicable to small, irregularly-shaped grains. Fortunately, gemologists retained the use of the refractometer which was ideal for non-destructive tests on cut and polished gems.
With the increasing popularity of the immersion method, a precision of routine refractive index measurements of + or -0.002 or + or -0.003 became acceptable although better precision was still achievable with double-variation methods. The general acceptance of such poor precision caused optical crystallography to lose effectiveness as a research tool in mineralogy.
More recent studies using spindle stage methods have disclosed that in research on mineral series wherein solid solution and/or order-disorder are involved, optical crystallography can be a powerful ally of X-ray crystallography. Such studies have produced the following advantages: (1) all principal indices and the dispersion of each were directly measurable from a single grain with a precision better than 0.0005; (2) the optical angle 2V of biaxial crystals was determinable, generally within a fraction of a degree, by the use of Bloss and Riess techniques and the computer program EXCALIBR (Bloss 1981); (3) the same crystal could then be studied by X-ray methods; and (4) the same crystal could be analyzed by electron microprobe.
The operative part of a jeweler's refractometer is a glass hemicylinder having a refractive index generally greater than 1.8 which sets the upper limit of the indices that are measurable. For convenience of reference, three rectangular Cartesian axes x, y and z will be defined herein such that x and y lie within the hemicylinder's polished plane with y coinciding with the axis of the cylindrical surface. An isotropic solid is placed with its cut and polished plane against the hemicylinder's xy plane and, to promote optical contact and adherence between the two planes, between them is placed a droplet of oil having a refractive index less than the hemicylinder but exceeding that of the refractive index of the solid. A cross-section through the solid specimen and the hemicylinder will show the critical angle phenomena that for a monochromatic light source permit the solid's refractive index, through measurement of the critical angle, to be determined. As is known, the image of the light-dark boundary is passed through a transparent scale and, after reflection by a mirror, is observed through a suitable lens system. The transparent scale can be calibrated to yield either the solid's refractive index for sodium light or its critical angle in fractions of a degree.
In the measurement of an anisotropic crystal, unless a polarizer is introduced into the light path, the single light-dark boundary associated with an isotropic solid will be replaced by two boundaries, namely a boundary between a light and not-so-light area as well as one between a not-so-light and a dark area. When in contact with the hemicylinder's xy plane, an anisotropic crystal, whether uniaxial or biaxial, exhibits two refractive indices which are not necessarily the principle indices. Their significance becomes apparent if it is considered that for an angle of incidence infinitessimally less than the critical angle, the refracted wave front travels within the crystal along a direction that practically coincides with the +x Cartesian axis. It is known that the two light vibrations associable with this wave front lie within this wave front and coincide with the major and minor axes of the ellipse formed by the intersection between the crystal's optical indicatrix and this wave front, which practically coincides with the yz plane. If it is assumed that the crystal is biaxial, its optical indicatrix possesses three mutually perpendicular, principal axes usually labelled X, Y and Z. If light vibrates parallel to X in the crystal, the crystal exhibits its smallest principal refractive index .alpha.; if parallel to Z it exhibits its largest principal refractive index .gamma.; and if parallel to Y it exhibits the intermediate principal index .beta..